Flange connections are not subjected to constant loads during service. Start‑up and shutdown operations, process fluctuations, and ambient temperature variations together constitute a cyclic load spectrum on the flange joint. The long‑term effect of this “breathing” load variation on sealing performance is far more complex than that of static conditions.
Gasket behaviour under cyclic loading is central to understanding sealing fatigue. Under cyclic bending loads, as the number of cycles increases, plastic strain accumulates in the gasket, and the leakage rate of the flange connection increases accordingly. The essence of this phenomenon is the ratcheting effect of the gasket material under repeated loading and unloading—even if the load amplitude of each cycle is modest, the accumulated plastic deformation reduces the effective gasket thickness and decreases the compression force.
The unique influence of thermal cycling lies in the additional stresses introduced by mismatched thermal expansion. Flanges, bolts, and gaskets have different coefficients of thermal expansion. When temperature cycles periodically, the components expand and contract by different amounts, causing the preload to fluctuate repeatedly through the cycle. Long‑term thermal cycling can lead to fatigue damage in the bolts and accelerated aging of the gasket. In life assessment of high‑temperature flange joints, the effect of thermal cycling is often more significant than that of pressure cycling.
The decisive influence of material selection on cyclic life cannot be overlooked. Under severe cyclic conditions, bolts for flange connections shall be made of alloy steel. The underlying reason for this requirement is that alloy steel offers better fatigue resistance and high‑temperature strength retention, maintaining more stable preload under cyclic loading. Gasket material selection is equally critical—flexible graphite gaskets offer better resilience under thermal cycling than many non‑metallic materials, enabling better compensation for微小 separation of the flange faces.
Assessment methods in practical engineering require a combination of finite element analysis and empirical data. By building a finite element model of the flange joint, the time history of stresses in each component under cyclic loading can be simulated, identifying potential fatigue critical locations. At the same time, leakage rate prediction models based on gasket compression deformation can calculate the evolution of leakage rate as the number of cycles increases. Combining the two provides an estimate of the sealing life of the flange joint under a given cyclic load spectrum.
Preventive maintenance strategies should be based on an understanding of the cumulative effects of cyclic loading. For flange connections subjected to frequent start‑up/shutdown or significant temperature fluctuations, the interval between periodic inspections should be shortened, with particular attention to the decay of bolt preload and the degree of gasket hardening. Preventive re‑tightening before bolt preload drops to a critical value is more economical and safer than addressing leakage after it occurs.
The “breathing” of a flange connection is a slow but continuous process. Neglecting cyclic loading effects often leads flanges to quietly approach sealing failure under seemingly normal operating conditions.
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